The present invention relates to a microcombustor for on-chip thermal management and sensor applications.
Most Microsystems currently use macroscopic power supplies and energy sources that are external to the Microsystems device. However, the use of macroscopic power supplies places severe limitations on the functionality of Microsystems for many applications. Therefore, a microsystem comprising an integrated; compact, and flexible power supply is highly desirable. Such an integral microscale power supply would typically need to store energy at a high density and discharge the stored energy at a high rate. A number of microscale power supply concepts have been considered, including microcombustors, electrochemical batteries, fuel cells, storage in magnetic or electric fields, storage as elastic strain energy, etc.
Microcombustors are becoming increasingly important for microsystems applications. Such microcombustors may be useful as Microsystems power supplies, for example, to convert chemical energy to electricity via thermoelectric or thermophotovoltaic generators or to produce hydrogen for fuel cells. In addition, the development of a small and stable on-chip microcombustor would permit the adaptation or translation of several very useful macroscopic devices into the microsystem domain, including on-chip flame ionization detectors (microFiDs), microreactors, micropropulsion, energy conversion and, importantly, heating and thermal management of microsystems. Microcombustors offer several advantages over other microscale power supply concepts for these applications. Microcombustion systems can provide on-demand, instantaneous power. Furthermore, hydrocarbon fuels offer approximately order of magnitude greater energy storage per mass than batteries. For example, the energy density of butane, including storage cylinder mass, is 50 times that of the best high-output batteries (e.g., nonrechargable LiMnO2 batteries). Hydrocarbon fuels are cheap and readily available and may present fewer environment concerns than batteries. Thus, a tiny fuel tank could replace several bulky batteries in hand-held microanalytical systems and could supply a microcombustor for efficient heating of essential components in a microsystem.
To sustain combustion in a microcombustor, the reactants must remain in the combustion chamber long enough to react and the temperature must not exceed the structural limits of the microcombustor materials. Reaction and residence times are effected by the choice of fuel, the fuel-to-air ratio, the size and geometry of the combustion chamber, and the gas-flow rate through the microcombustor. The scalability of combustion systems can be limited due to the increased surface-to-volume ratio at small combustor dimensions. In particular, thermal quenching due to heat losses to the walls and chemical quenching of reactive free radicals at surfaces become problematic as the dimensions of the combustor decrease, thereby limiting propagation of the combustion flame.
Prior art microcombustors having millimetric dimensions have been developed for power generation for microsystem devices. Cohen et al. in U.S. patent application Ser. No. 2001/0029974, discloses a microcombustor that relies on a toroidal counterflow heat exchanger to reduce heat loss from the combustor and to preheat the reactant gases. This microcombustor uses an external heater or an igniter internal to the heat exchanger to ignite combustion and is further configured with a thermoelectric material to generate electrical current. Masel et al., in U.S. Pat. No. 6,193,501, discloses a microcombustor having a combustion chamber that uses catalysts to get the reactants hot, ignited, and burning. Thermal barriers and an isolation cavity are used to minimize heat loss from a serpentine combustion chamber. Neither of these microcombustors use a microhotplate to minimize heat loss from the combustion chamber.
Microhotplates have been developed for micro-chemical reactors for partial oxidation synthesis and hydrogen reforming and for gas sensing. However, such microhotplates have typically been used to promote or sense reactions at the surface of the microhotplate and not to generate self-propagating combustion flames. See R. Srinivasan et al., xe2x80x9cMicromachined chemical reactors for surface catalyzed oxidation reactions,xe2x80x9d Tech. Digest 1996 Sol.-State Sensor and Actuator Workshop, pp. 15-18 (1996); L. R. Arana et al., xe2x80x9cA microfabricated suspended-tube chemical reactor for fuel processing,xe2x80x9d MEMS 2002, pp. 232-235 (2002); M. Gall, xe2x80x9cThe Si-planar-peilistor array, a detection unit for combustible gases,xe2x80x9d Sensors and Actuators B16. 260 (1993); R. P. Manginell et al., xe2x80x9cSelective, pulsed CVD of platinum on microfilament gas sensors,xe2x80x9d Tech. Digest 1996 Sol-State Sensor and Actuator Workshop, pp. 23-27 (1996); R. E. Cavicchi et al., xe2x80x9cMicrohotplate gas sensor,xe2x80x9d Tech. Digest 1994 Sol.-State Sensor and Actuator Workshop, pp. 53-56 (1994); and M. Zanni et al., xe2x80x9cFabrication and properties of a Si-based high sensitivity microcalorimetric gas sensor,xe2x80x9d Tech. Digest 1994 Sol.-State Sensor and Actuator Workshop, pp. 176-179 (1994).
Finally, microFID systems created by other groups have used micromachined nozzles to anchor an oxyhydrogen diffusion flame, which is essentially a miniaturization of existing technology. Zimmerman et al., xe2x80x9cMicro flame ionization detector and micro flame spectrometer,xe2x80x9d Sensors and Actuators B 63, 159 (2000) and Zimmerman et al., xe2x80x9cMiniaturized flame ionization detector for gas chromatography,xe2x80x9d Sensors and Actuators B 83, 285 (2002) describe a miniaturized flame ionization detector that comprises a micro burner unit with a nozzle diameter of less than 100 xcexcm to produce a stable miniature flame. Oxyhydrogen flow rates on the order of 35 ml/min were required for flame stabilization in this design.
There remains a need for an integrated, flexible, and efficient microcombustor that can be used for power generation, heating and thermal management of on-chip Microsystems, and for other sensor applications. Unlike the prior art, the present invention satisfies this need by providing a microcombustor comprising a microhotplate with a very low heat capacity and thermal conductivity to minimize heat loss from the combustion chamber and a surface catalyst for flame ignition and stabilization.
The microcombustor of the present invention combines a microhotplate and catalyst materials for sustained combustion on the microscale. The microhotplate comprises a thin-film heater/thermal sensor patterned on a thin insulating support membrane that is suspended from its edges over a substrate frame. This microhotplate has very low heat capacity and thermal conductivity and is an ideal platform for heating catalytic materials placed on the surface of the support membrane. The free-standing platform used in the microcombustor mitigates large heat losses arising from large surface-to-volume ratios typical of the microdomain, and, together with the heated catalyst, permits combustion on the microscale.
The heated catalyst enables flame stabilization, even in spaces with large surface/volume ratios; permits combustion with lean fuel/air mixtures; extends a hydrocarbon""s limits of flammability; and lowers the combustion temperature. Surface oxidation, flame ignition, and flame stabilization have been achieved for hydrogen and hydrocarbon fuels premixed with air. Flame stabilization via catalytic surfaces permits stable combustion at hydrogen flows less than 5 ml/min and under lean conditions. In addition to providing for stable flames in the microdomain, the microcombustor expands the limit of flammability (LoF) for many hydrocarbon fuels, as compared with diffusion flames. For example, the LoF of the microcombustor for natural gas in air is 1-35%, as compared to the 4-16% typically observed. The LoF for hydrogen, methane, propane and ethane are likewise expanded. This expanded LoF has important consequences for microanalytical systems: not only is the energy density of combustible gases relatively high, but the microcombustor also allows for lean burning at low flows and at temperatures less severe than with diffusion flames. The reduced operating temperatures enable a longer system lifetime and the reduced fuel consumption enables smaller fuel supplies, both of which are especially important for portable applications.
The microcombustor can be used for on-chip thermal management of Microsystems. The microcombustor of the present invention provides heat densities of greater than 35 mW/xcexcm2 for heating microsystems.
The microcombustor can be used for other sensor applications in microanalytical systems. A micro-scale flame ionization detector (microFID) is provided by coupling an electrometer circuit with miniature electrodes in the combustion chamber. The microFID of the present invention uses catalytically stabilized combustion on a microhotplate for the flame ionization detection of hydrocarbon ionization from the combustion of fuels. The catalytically stabilized flame con operate over broader combustion limits and at reduced temperatures compared to conventional FIDs. Therefore, the microFID can be used with premixed fuels. The microFID can be used to determine fuel carbon content. For example, the detection of approximately 1-3% of ethane in hydrogen/air is achieved using premixed fuel and a catalytically-stabilized flame. Because the microFID has high sensitivity and selectivity with a minimum response time, it may be useful for real-time monitoring of analytes eluted from a gas chromatography column.